Lint/FormatParameterMismatch ignores % if LHS or RHS is not a literal

If the LHS is not a literal, it could evaluate to a format string with any
number of arguments, so we can't count an offense for that. On the other hand,
if the RHS is not a literal, it could evaluate to an array with any number
of arguments, so we can't count an offense for that either.

Fixes rubocop#2307.

CHANGELOG.md

@@ -41,6 +41,7 @@
 * [#2331](https://github.com/bbatsov/rubocop/issues/2331): Do not register an offense in `Performance/Size` for `count` with an argument. ([@rrosenblum][])
 * Handle a backslash at the end of a line in `Style/SpaceAroundOperators`. ([@lumeet][])
 * Don't warn about lack of "leading space" in a =begin/=end comment. ([@alexdowad][])
+* [#2307](https://github.com/bbatsov/rubocop/issues/2307): In `Lint/FormatParameterMismatch`, don't register an offense if either argument to % is not a literal. ([@alexdowad][])
 
 ## 0.34.2 (21/09/2015)
 

lib/rubocop/cop/lint/format_parameter_mismatch.rb

@@ -37,7 +37,10 @@ def offending_node?(node)
               false
             else
               num_of_format_args, num_of_expected_fields = count_matches(node)
-              num_of_expected_fields != num_of_format_args
+
+              num_of_format_args != :unknown &&
+                num_of_expected_fields != :unknown &&
+                num_of_expected_fields != num_of_format_args
             end
           else
             false
@@ -79,22 +82,36 @@ def heredoc?(node)
           args.loc.expression.source[0, 2] == SHOVEL
         end
 
+        def literal?(node)
+          node.int_type? ||
+            node.str_type? ||
+            node.sym_type? ||
+            node.float_type? ||
+            node.dstr_type? ||
+            node.dsym_type?
+        end
+
         def count_matches(node)
           receiver_node, _method_name, *args = *node
 
           if (sprintf?(node) || format?(node)) && !heredoc?(node)
             number_of_args_for_format = (args.size - 1)
             number_of_expected_fields = expected_fields_count(args.first)
-          elsif percent?(node)
-            first_child_argument = args.first
+          elsif percent?(node) && args.first.array_type?
+            number_of_expected_fields = expected_fields_count(receiver_node)
 
+            first_child_argument = args.first
             if first_child_argument.array_type?
               number_of_args_for_format = args.first.child_nodes.size
-              number_of_expected_fields = expected_fields_count(receiver_node)
-            else
+            elsif literal?(first_child_argument)
               number_of_args_for_format = 1
-              number_of_expected_fields = expected_fields_count(receiver_node)
+            else
+              # non-literals might evaluate to an Array, or they might not
+              # so we can't tell just how many format args there will be
+              number_of_args_for_format = :unknown
             end
+          else
+            number_of_args_for_format = number_of_expected_fields = :unknown
           end
 
           [number_of_args_for_format, number_of_expected_fields]
@@ -113,6 +130,7 @@ def format_method?(name, node)
         end
 
         def expected_fields_count(node)
+          return :unknown unless node.str_type?
           node
             .loc
             .expression

spec/rubocop/cop/lint/format_parameter_mismatch_spec.rb

@@ -133,6 +133,30 @@
     expect(cop.offenses).to be_empty
   end
 
+  context 'when multiple arguments are called for' do
+    context 'and a single variable argument is passed' do
+      it 'does not register an offense' do
+        # the variable could evaluate to an array
+        inspect_source(cop, 'puts "%s %s" % var')
+        expect(cop.offenses).to be_empty
+      end
+    end
+
+    context 'and a single send node is passed' do
+      it 'does not register an offense' do
+        inspect_source(cop, 'puts "%s %s" % ("ab".chars)')
+        expect(cop.offenses).to be_empty
+      end
+    end
+  end
+
+  context 'when format is not a string literal' do
+    it 'does not register an offense' do
+      inspect_source(cop, 'puts str % [1, 2]')
+      expect(cop.offenses).to be_empty
+    end
+  end
+
   it 'ignores percent right next to format string' do
     inspect_source(cop, 'format("%0.1f%% percent", 22.5)')
     expect(cop.offenses).to be_empty